Sales of probiotic products in the community generate billions of dollars worldwide, yet many of the health claims made by the industry lack a rigorous scientific basis. Studies on the effectiveness of commercially prepared probiotic products have produced varying results and opinions are divided on the clinical benefits and risks of probiotics, which are likely to be significant in some vulnerable patient groups

What is a probiotic?

A product is considered to be probiotic if it contains microorganisms that provide benefit when ingested. There are
numerous types of microorganism that can be found in probiotic products, e.g. Lactobacillus acidophilus, L.
rhamnosus and Bifidobacterium lactis (bacteria used in the production of dairy products) and Saccharomyces
boulardii (a strain of yeast). Supplements containing freeze-dried microorganisms, marketed as being probiotic,
are available over-the-counter (OTC) at pharmacies, “health” stores and supermarkets. Live cultures of microorganisms
are also sometimes added to commercially produced foods, e.g. probiotic yoghurt. However, whether or not probiotic products
actually contain live microorganisms at the time of consumption, and even assuming they do, it is largely unknown whether
any of these microorganisms are able to form beneficial living colonies in the person taking them.

The majority of the potential health benefits attributed to probiotics are thought to occur in the host’s gastrointestinal
tract. These may include: improved digestion and nutrient absorption, enhanced immune function and re-colonisation of
the gastrointestinal tract following a disturbance in gut microflora, as can occur after treatment with antibiotics.

The concentration of microorganisms in a probiotic is generally reported in colony forming units (CFU). Patients who
are taking antibiotics are reported to need to ingest between five and 40 billion CFU/day of a probiotic organism in order
to experience any benefit.1

Do probiotics protect against antibiotic-associated diarrhoea?

When ingested in sufficient quantities, probiotics do appear to reduce the risk of antibiotic-associated diarrhoea in
children and adults. This protective ability appears to broadly extend across different types of antibiotic and different
probiotics.

A meta-analysis of 16 studies, including over 3400 young people aged between two weeks and 17 years, reported a number
needed to treat (NNT) of seven patients for the probiotics Lactobacillus rhamnosus or Saccharomyces
boulardii to prevent one case of antibiotic-associated diarrhoea.1 Another meta-analysis of 34 randomised
controlled trials that included over 4100 patients assessed regimens of: bifidobacteria, lactobacilli, enterococci, streptococci, Saccharomyces
boulardii and combinations of these probiotics.5 The NNT for the probiotics to prevent one episode
of antibiotic-associated diarrhoea was eight patients.5 Probiotics were most effective at preventing diarrhoea
in patients taking antibiotics for H. pylori with a NNT of five.5

The duration of probiotic treatment varied between the studies, but a broadly representative regimen would be commencing
probiotics on the day that antibiotic treatment was begun, and continuing for at least as long as the course of antibiotic
treatment and up to one week after.5 The authors concluded that there was a relatively consistent protective
effect across the range of probiotics studied, that this effect was seen in paediatric and adult populations and was maintained
across different antibiotics and different indications for the antibiotic use.5

Antibiotic use is associated with a range of gastrointestinal adverse effects

By altering the gut microflora, antibiotics can cause a range of adverse effects, including Clostridium difficile colitis
and antibiotic-associated diarrhoea. Diarrhoea is reported to occur in 5 – 30% of patients prescribed antibiotics,2 and
is most likely to occur with broad spectrum antibiotics, e.g. penicillin derivatives, cephalosporins and clindamycin.3 The
use of antibiotics is also associated with approximately a four-fold increased risk of vaginal candidiasis in females.4 Some
patients who are prescribed antibiotics will consider taking adjunctive probiotics in order to reduce their risk of experiencing
antibiotic-associated adverse effects.

Do probiotics protect against vaginal candidiasis?

In contrast to their effectiveness in preventing antibiotic-associated diarrhoea, there is evidence that probiotics
are not protective against post-antibiotic vulvovaginal candidiasis. A study of 235 non-pregnant females aged 18 – 50
years found that preparations of Lactobacillus taken orally or vaginally, or by both routes, did not reduce
the risk of developing post-antibiotic vulvovaginal candidiasis.6

But are probiotics safe?

The issue of probiotic safety is complicated by two factors. Firstly, adverse effects are frequently poorly reported
in studies, as the assumption is often made that probiotics are safe. A meta-analysis of trials that did include adverse
events found no statistical difference in the incidence of symptoms such as rash, vomiting or other gastrointestinal effects
and cough between treatment and control groups.1 Secondly, the range of bacteria that are included in probiotic
products is vast. It is therefore not possible to comment conclusively on the safety of all probiotics. This makes it
very difficult to balance the risks and the benefits of probiotics.

A review focusing on Lactobacilli discussed three theoretical safety concerns associated with the use of probiotics:7

The possibility of infectious disease

Toxicity in the gastrointestinal tract

The possible transfer of antibiotic resistance from probiotic bacteria to microflora in the gastrointestinal tract

Lactobacillus bacteria species that are present in probiotics can cause bacteraemia and endocarditis; immunosuppressed
people appear to be at the greatest risk.7 Although rare, there have been a number of cases of bacteraemia
or fungemia related to the use of probiotics.7 However, epidemiological data does not indicate any increase
in these infections associated with the use of probiotics.7 Based on the limited evidence available, the
risk of infections related to probiotic use is very small and appears to be higher in young children with short gut syndrome
and older patients who are immunosuppressed, e.g. following HIV infection, or those who have had an organ transplant.7

There are a range of toxic effects that probiotics could theoretically cause in the gastrointestinal tract, including:
malabsorption due to disruption of bile salts, lactate production resulting in lactic acidosis, modification of immune
responses, degradation of intestinal mucus.7 However, there is little evidence suggesting that any of these
theoretical effects are likely to be clinically relevant to patients who do not have underlying gastrointestinal abnormalities,
e.g. short gut syndrome, although in these patients the consequences could be serious.7

Bacteria within the human intestine are able to share genes that confer antibiotic resistance between themselves and
they are also able to acquire or donate these resistance genes to other bacteria within the gastrointestinal tract.8 Some
probiotic lactic acid bacteria have been isolated that are resistant to certain antibiotics, e.g. erythromycin, vancomycin,
tetracycline, and probiotic bacteria have also been shown to be able to transfer antibiotic resistance to other species
of bacteria in vitro.8 This has lead to a safety concern that because cultures used in the production
of probiotics are not routinely screened for antibiotic resistance, it may be possible for potentially pathogenic bacteria
within the gastrointestinal tract to acquire antibiotic resistance from probiotic bacteria.8 However, as
is the case with many aspects of probiotics, it is currently unknown how likely this possibility is.

More extensive research, and consistent reporting of adverse reactions via pharmacovigilance monitoring programmes,
would be expected to provide better information on the likelihood, nature and seriousness of adverse events with probiotics.